Two transistor model is used to explain the principle of operation of the thyristor in a very simple way. Since a thyristor is a four-layered PNPN device, it can be considered as a combination of two transistors, one transistor as PNP and the other transistor as NPN. A two-transistor model is obtained by separating the two middle layers of the thyristor into two parts as shown in the below figure.

In this model, base current I_{b1} of transistor Q_{1} is equal to the collector current I_{c2} of transistor Q_{2} and vice-versa. The equivalent circuit of the two transistor analogy is shown below.

#### From the above equivalent circuit, the base and collector currents of the transistors Q_{1} and Q_{2} can be written as,

*I*

_{b1}= I_{c2}and I_{c1}= I_{b2}…(1)Also, from the above figure, we have, cathode current is the sum of anode current and gate current.

*I*

_{k}= I_{a}+ I_{g}…(2)We know that, collector current I_{c} is related to emitter current I_{e} as,

*I*

_{c}= Î±I_{e}+ I_{cbo}Where,

- Î± = Common-base current gain â‰ƒ I
_{c}/I_{e} - I
_{cbo}= Leakage current of collector-base.

Hence, in the given equivalent circuit,

*I*

_{c1}= Î±_{1}I_{e1}+ I_{cbo1}and*I*

_{c2}= Î±_{2}I_{e2}+ I_{cbo2}…(3)But, I_{e1} of Q_{1} = I_{a} and I_{e2} of Q_{2} = I_{k}. Subsituting the values of I_{e1}, I_{e2} in equation 3, we get,

*I*

_{c1}= Î±_{1}I_{a}+ I_{cbo1}and*I*

_{c2}= Î±_{2}I_{k}+ I_{cbo2}…(4)The sum of two collector currents is equal to an anode current. Therefore,

*I*

_{a}= I_{c1}+ I_{c2}…(5)Substituting equation 4 in equation 5, we get,

*I*

_{a}= Î±_{1}I_{a}+ I_{cbo1}+ Î±_{2}I_{k}+ I_{cbo2}…(6)#### From transistor analysis, we have,

*I*

_{e1}= I_{b1}+ I_{c1}*I*

_{b1}= I_{e1}– I_{c1}…(7)Substituting equation 2 in equation 6, we get,

If the leakage currents I_{cbo1} and I_{cbo2} of transistors Q_{1} and Q_{2} are negligibly small, then,

From the above equation, it can be analyzed that, if Î±_{1} + Î±_{1} = 1, then anode current I_{a} becomes infinity and hence thyristor enters into conduction state from non-conduction state. In other words, if I_{g} reaches a value such that Î±_{1} + Î±_{2} = 1, then SCR is turned ON and this action is called the Regenerative action.

#### The regenerative action i.e., the condition Î±_{1} + Î±_{1} = 1 can be obtained in the following ways,

- The increase in temperature increases the leakage current by which the required above condition can be obtained to turn on the SCR.
- The SCR can be also turned ON by increasing Î±
_{1}and Î±_{2}. Since Î±_{2}depends on (I_{a}+ I_{g}) and Î±_{1}on I_{a}, the current gain of the transistor Q_{2}can be increased without affecting voltage V_{a}and current I_{a}by injecting the current I_{g}through the gate in the same direction of I_{a}. Thus the above condition can be obtained.